1. Field of the Invention
The present invention is in the field of semiconductor structures. The present invention is further in the field of semiconductor structures of variable capacitance devices. Particularly, it relates to a MOS type variable capacitance device for semiconductor circuits. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into larger integrated circuits.
2. Brief Description of Related Art
Semiconductor capacitors are one of the fundamental components for integrated circuits. A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed under the influence of DC bias voltages. Variable capacitors are often used in L-C circuits to set the resonance frequency, e.g. to tune a radio (therefore they are sometimes called tuning capacitors), or as a variable reactance, e.g. for impedance matching in antenna tuners.
A voltage-controlled capacitor is well known in the art as “varactor”, in which the thickness of a depletion region formed in a pn-junction diode is varied by changing a reverse bias voltage to alter the junction capacitance. Any junction diode exhibits this effect (including pn-junctions in transistors), but devices used as variable capacitance diodes are designed with a large junction area and a doping profile specifically chosen to maximize the capacitance tuning range.
Their use is limited to low signal amplitudes to avoid obvious distortions as the capacitance would be affected by the change of signal voltage, precluding their use in the input stages of high-quality RF communications receivers, where they would add unacceptable levels of inter-modulation. At VHF/UHF frequencies, e.g. in FM Radio or TV tuners, dynamic range is limited by noise rather than large signal handling requirements, and varcaps are commonly used in the signal path. Furthermore an extremely high value of capacitance cannot be obtained even with a reverse bias because the reverse-biased saturation current is not exactly equal to zero.
Varcaps are used for frequency modulation of oscillators, and as reported in Miyagi et al. (U.S. Pat. No. 7,403,140) to make high-frequency voltage controlled oscillators (VCOs), the core component in phase-locked loop (PLL) frequency synthesizers that are ubiquitous in modern communications equipment. These components are intended for antenna impedance matching in multi-band GSM/WCDMA cellular handsets and mobile TV receivers that must operate over wide frequency ranges such as the European DVB-H and Japanese ISDB-T mobile TV systems.
Several prior art attempts to improve varactors performance, so as to effectively obtain high capacitance density and a linear dependence of the capacitance value over a wide range of control voltages, have been documented. In particular, an interesting solution is reported in Ogawa et al. (U.S. Pat. No. 7,622,760) where the synthesis of two MOS capacitor is used to obtain a good linearity over a wide range relative to the DC control voltage. However, the prior art described above discloses a varactor that is still a two terminal device, and its capacitance is varied by modulating the DC voltage between its two terminals. This leads to the disadvantage that the AC voltage is superimposed upon the DC control value, and therefore the capacitance value is distorted by the AC voltage.
There is therefore a need for a novel variable capacitor with at least three terminals, where at least one control terminal separated from the capacitance terminals is added to the component to introduce capacitance variability without interfering with the voltage across the main terminals of the capacitor. The novel structure should allow the control of the capacitance without overlapping the DC control voltage with the AC signal thus avoiding the distortion of the capacitance value during the circuit operation.
A similar approach, where a three terminal MOS varactor is used, is disclosed in Kohashi (U.S. Pat. No. 3,829,743). In this patent the author describes a variable capacitance device having a thin film of dielectric material in which the area of an equivalent plate electrode is varied by changing the voltage of the control terminal or under the influence of radiations. Referring to the drawings in Kohashi and more particularly to FIG. 1 and FIG. 2 of the cited patent, the variable capacitance device comprises a pn-junction diode placed directly above the dielectric film and a source of DC voltage. A lead wire made of gold or aluminum is placed in ohmic contact with an end surface of each of the p and n regions.
One lead is connected to the movable contact of a double-throw switch. The double-throw switch has two fixed contacts connected to two batteries, which in turn are connected together to the other lead placed in electrical contact with the n region. As shown, a thin film of high-insulation, low-dielectric-loss dielectric material is deposited on the side surface of the diode perpendicular to the junction. The described device uses the voltage between the n and the p regions of the pn-junction to modulate the depletion region above the oxide in order to change the overlap surface between the p and n regions with the metal plate under the oxide layer. FIG. 3 of the cited patent shows the structure resulting by the parallel of two structures as presented in FIG. 1.
The described structures can be used only for discrete components. As underlined by the author in the patent description, the structures are not suitable for integrated circuits. The integrated version of the structure shown in FIG. 1 is reported in FIG. 4. In this case, as in the previous one, both the depletion regions in the p and n regions are used to modulate the capacitance, which lead to a difficult control of the device performance and capacitance-voltage relation (the process variations of the p-region sum up with the one of the N region and to the variability on the position of the pn-junction).
Furthermore, in all these structures, the DC voltage is applied between one terminal of the capacitance and a region directly in contact with the capacitance dielectric layer overlapping the metal terminal 27 in FIG. 1 (or 45 in FIG. 3), causing a distortion of the capacitance value due to the modulation of the MOS capacitance. By varying the DC voltage between the p and n regions, also the DC voltage drop between one of these two regions and the metal terminal changes, and that causes an enhancement or depletion of the semiconductor surface affecting the capacitance value.
The last structure of interest disclosed in Kohashi is the one illustrated in FIG. 14 of the cited document. In this case the variable capacitance is the resulting synthesis of the series of the capacitances of the pn-junctions and a MOS structure. The capacitor terminals 190 and 193 are coupled through a p+/n junction and the MOS capacitance. In this case the resulting capacitance and its range of variation are therefore very low. Furthermore, in this configuration the capacitance depends also on the thickness of the depletion regions of the two p+/n junction as in conventional diode based varactors, leading to high distortion of the capacitance value. Finally, it is important to notice that none of the structures described in Kohashi has a linear relation between the control voltage and the capacitance value.
Ideally, an analog component, where the relation between the control voltage and the capacitance value is linear, would be desirable for adaptive linear control of feedback systems, however this is difficult to achieve because many factors contribute to the non uniform variation of capacitance with the control voltage. Nevertheless a more digital approach, where the capacitance value is abruptly varied between its lowest and largest value, is desirable as well, because many small digital capacitors coupled in a parallel to form a capacitive array may result in a larger capacitor where its value is selected and modified in a discrete fashion.
The present invention is simple, much less sensitive to process variations with respect to the structures described above, it is suitable for integrated circuits and presents a high capacitance density value. In the present invention the variation of the DC control voltage, in theory, does not affect the voltage applied between the two capacitance plates, therefore enabling an excellent control of the device characteristic. Furthermore, the present invention exhibits high Q (quality factor) value of the component. All these characteristics are extremely important for the practical implementation of the present invention and clearly distinguish the present invention from the varactors devices used nowadays in the integrated-electronic industry and in particular in radio-frequency applications.
It is a purpose of the present invention to describe a novel structure of a semiconductor controllable capacitor suitable for integrated circuits with at least three terminals, simple, cost effective and immune to process variations, which offers the advantages of high capacitance per unit area, wide control range, high Q and very low distortion of the RF signal applied to the capacitor.